In the last decade, the automotive industry has seen a rise in demand for electrical energy especially for light vehicles. This trend is the result of an increase in the amount of electrical comfort and safety features in modem vehicles. Another contributing factor is the increasing electrification of formerly purely mechanical functions as can be seen by all the upcoming x-by-wire systems.
The development in electrical power generation has been struggling to keep up with this demand. However, the entrance barriers associated with new concepts for power generation systems have so far been too high to allow a quick proliferation. Nevertheless, there have been many developments in many research labs of the automotive industry directed to alternative generation systems that provide sufficient power to feed most of the additional functions. These systems include dual output alternators, switch-mode rectification, starter-generators, and active rectifiers.
Of these systems the starter-generator has received the highest publicity. The starter-generator system, however, has the highest development cost, tooling cost and implementation cost (in terms of re-packaging the engine compartment). Also, dual output alternators and switch-mode rectification both have technical disadvantages.
Active rectification has been discussed for a long time, but only recently has the development in high end FETs made the concept feasible.
Active rectification is simply the replacement of passive diodes with active FETs. These FETs are switched ON/OFF to perform the rectification function similar to the “automatic” switching initiated by the forward voltage drop of a diode. For example, in active synchronous rectification the FETs are turned ON/OFF according to the frequency of the stator phases.
For active rectification detection driver circuits are required to generate the gate voltage (derived from the stator phase voltages) necessary for switching a FET ON/OFF, thereby rectifying the current like a diode. Active rectification, however, results in much lower losses since the voltage drop across a FET is only a fraction of the forward voltage drop of a diode. As a result, less energy is wasted, which allows for higher output.
Typically, active rectification is more difficult at high rpm due to timing issues. However, at low rpm rectification is more effective.